In various industrial, technological and scientific fields, it is frequently necessary to determine instantaneous values of sound propagation speed or velocity. For example, many systems depend upon the sound propagation velocity in air which varies nonlinearly as a function of the temperature and, to date, instantaneous values of the sound propagation velocity in air could not be obtained in a simple and convenient manner or with relatively simple apparatuses. Indeed, repeated or continuous monitoring of the sound propagation velocity in air subject to temperature fluctuations has not been possible.
To measure sound propagation velocity, it is known to provide a glass tube filled with air which has a closure at an end of the air-filled column, which is subject to movement and to which a predetermined acoustical frequency (a harmonic of the fundamental) can be applied so that the mechanical displacement results in a resonance wave within the tube. The resonance is established by changing the effective length of the column by movement of this member and the longitudinal wave established in the air column can be made visible by the use of cork dust or powder which develops nodes or condensation patterns separated by rarefaction patterns.
The wavelength can be measured from the cork powder patterns and the sound propagation velocity calculated.
In German patent document DE-OS No. 29 43 766 and the corresponding U.S. Pat. No. 4,303,853, issued Dec. 1, 1981, commonly assigned with the present case, a method of and an apparatus for determining the impact site of a bullet upon a target has been described. In principle, this system utilizes the time-staggered pick-up of acoustic signals by a plurality of spaced-apart acoustoelectrical transducers to provide an indication of the location of the point of impact upon the target.
The location, of course, can only be determined with precision if the sound propagation velocity of the medium between the target surface and these transducers is known with equal or greater precision.
In closed target assemblies of the aforedescribed type, it has been shown experimentally that the temperature is not constant but continuously varies and that the sound propagation velocity is a nonlinear response to this continuously fluctuating temperature. Because of this phenomenon, the mathematics for calculating the impact location are complex and it is not uncommon, unless significant effort is made to minimize the temperature change, for the precision of the measurement to lie below the minimum standards of the UIT (Union Internationale de Tir).
Since methods and devices for measuring the said propagation velocity, which would have enabled the system of the aforementioned patent to be utilized with precision, were not then available, that patent describes means for minimizing temperature fluctuations and thereby maintaining the sound propagation velocity practically constant.
For such apparatus and other applications in the field of technology, industry and research, it is important to have means for determining instantaneous values of sound propagation velocity.